The present invention relates to alarm, warning, or control system and, more particularly, to systems wherein information may be transmitted from one point to another under conditions wherein either or both of the points may be in motion and the action which should be taken at the receiving point may be a function of several factors including: the physical proximity of the two points; the magnitude and direction of the relative velocity between the two points; the character of the transmitted signal and other factors. For example, if the signal radiating source comprises a moving train and the signal receiver comprises equipment for operating grade crossing signals, it will be apparent that the direction of motion of the train relative to the grade crossing is important relative to the decision to provide signals to interrupt traffic at the grade crossing. That is, if the train is approaching the grade intersection, traffic should be interrupted; but if the train is receding from the grade intersection, traffic need not be interrupted. Similar techniques could be used in conjunction with ambulances and fire trucks, or other vehicles, to provide signals for controlling traffic lights at intersections as they are approached. There are numerous other situations wherein it would be desirable to provide signals of different character depending upon the changing physcial relationships between the transmitting and receiving stations. For example, the technique could be employed in a bus transportation system to help maintain uniform spacing between buses and to avoid bunching.
Other situations wherein it is desirable to transmit a signal from one location, receive it at another location and then produce a new signal indicative of selected physical relationships between the two locations will readily occur to those familiar with the physical characteristics and circumstances of their application.
In many applications, voice communication by radio provides very satisfactory results and information exchange. However, such techniques require human attendance, bulky and expensive equipment, and frequently a license for use. In addition, voice radio communication cannot provide information indicative of the relative motion between a moving transmitting station and a moving receiving station without lengthy conversational exchanges concerning their locations.